This invention relates to electronic digital communication systems and more particularly to receivers in wireless communication systems.
Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS and WCDMA standards. This application focusses on WCDMA systems for simplicity, but it will be understood that the principles described in this application can be implemented in other digital communication systems.
WCDMA is based on direct-sequence spread-spectrum techniques, with pseudo-noise scrambling codes and orthogonal channelization codes separating base stations and physical channels (terminals or users), respectively, in the downlink (base-to-terminal) direction. Since all users share the same radio frequency (RF) resource in CDMA systems, it is important that each physical channel does not use more power than necessary. This is achieved by a transmit power control (TPC) mechanism, in which, among other things, base stations send TPC commands to users in the downlink (DL) direction and the users implement the commands in the uplink (UL) direction and vice versa. The TPC commands cause the users to increase or decrease their transmitted power levels by increments, thereby maintaining target signal-to-interference ratios (SIRs) for the dedicated physical channels (DPCHs) between the base stations and the users. The DPCHs include dedicated physical data channels (DPDCHs) and dedicated physical control channels (DPCCHs) in the UL and the DL. A DPDCH carries higher-layer network signaling and possibly also speech and/or video services, and a DPCCH carries physical-layer control signaling (e.g., pilot symbols/signals, TPC commands, etc.). WCDMA terminology is used here, but it will be appreciated that other systems have corresponding terminology. Scrambling and channelization codes and transmit power control are well known in the art.
FIG. 1 depicts a communication system such as a WCDMA system that includes a base station (BS) 100 handling connections with, in this example, four mobile stations (MSs) 1, 2, 3, 4. In the downlink, BS 100 transmits to each mobile at a respective power level, and the signals transmitted by BS 100 are spread using orthogonal code words. In the uplink, MS 1-MS 4 transmit to BS 100 at respective power levels. Each BS, which is called a Node B in 3GPP parlance, in the system serves a geographical area that can be divided into one or more cell(s). The BSs are coupled to corresponding radio network controllers (RNCs, not shown in FIG. 1) by dedicated telephone lines, optical fiber links, microwave links, etc. An RNC directs MS, or user equipment (UE), calls via the appropriate BSs, and the RNCs are connected to external networks such as the public switched telephone network (PSTN), the Internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
WCDMA is designed to operate at low signal-to-noise ratios (SNRs), and therefore the WCDMA algorithms, for instance, the SIR estimators and automatic frequency control (AFC) algorithms, are designed for such scenarios. For example, the SIR estimation algorithm, which is used in the transmit power control (TPC) scheme to achieve sufficient quality of service (QoS), is designed to be used at low SIRS. QoS is often quantified by block error rate (BLER). It will be understood that, in WCDMA systems (and other communication systems that employ direct-sequence (DS) spread-spectrum techniques), the noise (N) includes thermal noise and interference because the spreading of the signals makes interference signals appear noise-like (i.e., spread out in frequency and with a level in the noise floor) due to the interference signals' “wrong” spreading codes.
The SIR is used for inner loop power control because it is assumed to have an almost one-to-one mapping to the BLER. Outer loop power control, which operates with a slow response rate, is also included in WCDMA in order to compensate for residual mismatch between SIR and BLER. Power control and SIR-to-BLER mapping are well known in the art, and are described in, for example, Louay M. A. Jalloul et al., “SIR Estimation and Closed-Loop Power Control for 3G”, IEEE pp. 831-835 (2003).
In such a communication system, the BS transmits predetermined pilot symbols on the UE's DPCH. The BS also transmits pilot symbols on a common pilot channel (CPICH), and a UE typically uses the CPICH pilot symbols in estimating the impulse response of the radio channel to the BS. It will be recognized that the UE uses the CPICH pilots for channel estimation, rather than the DPCH pilots, due to the CPICH's typically higher SNR, but the UE still uses the DPCH pilots, mainly for SIR estimation, i.e., for DL power control.
It is also known that a better SIR estimator gives better receiver performance, measured as the amount of power needed for a given BLER target, with lower power needed being better. In order to improve the SIR estimator in WCDMA, one can use the CPICH for the I estimate and use only the DPCH pilots for estimating the S part of the SIR. This is described in, for example, U.S. Patent Application Publication No. 2005/0094816 by Lindoff et al. for “Interference Estimation in CDMA Systems Using Alternative Scrambling Codes”. The following five equations express such a SIR estimator.
For the S, the wanted signal estimate SiDPCH is given by:SiDPCH=|ĥDPCH,i|2,   Eq. 1where:
                                          h            ^                                DPCH            ,            i                          =                              1                          n              p                                ⁢                                    ∑                              k                =                1                                            n                p                                      ⁢                                          u                k                p                            ⁢                                                y                                      DPCH                    ,                    i                                    *                                ⁡                                  (                  k                  )                                                                                        Eq        .                                  ⁢        2            and nP is the number of DPCH pilot symbols ukP per slot, yDPCH,i(k) is the de-spread DPCH pilot symbol at the time instant k for rake finger i, and * means complex conjugate.
For the I, the interference signal estimate IiDPCH is given by:
                              I          i          DPCH                =                                            SF              C                                      SF              D                                ⁢                      I            i            CPICH                                              Eq        .                                  ⁢        3            where SFC is the spreading factor for the channel, e.g., the CPICH, used to calculate the I estimate, and SFD is the spreading factor for the channel, e.g., the DPCH, to which the I estimate is to be translated, in case these are different channels, and:
                              I          i          CPICH                =                              1                                          N                C                            -              1                                ⁢                                    ∑                              k                =                1                                            N                C                                      ⁢                                                                                                                      y                                              CPICH                        ,                        i                                                              ⁡                                          (                      k                      )                                                        -                                                                                    h                        ^                                                                    CPICH                        ,                        i                                                              ⁢                                          u                      k                      CPICH                                                                                                  2                                                          Eq        .                                  ⁢        4            where ukCPICH is the CPICH pilot symbol k, ĥCPICH,i is the CPICH channel estimate for tap i, yCPICH,i(k) is the de-spread CPICH pilot symbol at time instant k for rake finger i, and NC is the number of pilot symbols per slot for the channel used to obtain the I estimate. SFC is typically 256 and the CPICH has ten pilot symbols per slot in a WCDMA communication system. In this example, the CPICH symbols in one slot (i.e., 10 symbols) are used to determine the I-estimate. It will be appreciated that different numbers of symbols may be used, and different communication systems may have different numbers of symbols in a slot.
For the SIR estimate SIREST:
                              SIR          EST                =                              ∑                          i              =              1                                      n              f                                ⁢                                    S              i              DPCH                                      I              i              DPCH                                                          Eq        .                                  ⁢        5            where nf is the number of rake fingers.
In laboratory tests and benchmark scenarios, good signal quality is often assumed, which is to say that the terminal operates with good SNR. Also in such cases, good terminal behavior is needed, which means that the needed downlink power should be small if the SNR of the CPICH is high. A “non-good” terminal behavior is described below, involving long power control loop transients. In such scenarios, the residual frequency error, which is the frequency error remaining after the AFC has corrected the tuning of the receiver, affects the I-estimate more than it affects the BLER. It will be appreciated that a SIR-to-BLER mapping that is heavily dependent on the interference level changes the SIR reference value in the outer loop power control, and due to the slow response of the outer loop power control, long transients occur, in which the downlink power level is set too high. Thus, erroneous SIR estimates are obtained in these scenarios.